Scientists have crushed two layers of ultracold magnetic atoms to within 50 nanometers of each other – 10 times closer than in previous experiments – revealing strange quantum effects never seen before.
The extreme proximity of these atoms will allow researchers to probe quantum interactions on this scale for the first time and could lead to important advances in the development of superconductors and quantum computersthe scientists reported in a new study published May 2 in the journal Science.
Unusual quantum behaviors begin to emerge at ultralow temperatures as atoms are forced into their lowest possible energy state. “In the nanokelvin regime, there is a type of matter called Bose Einstein condensate [in which] all particles behave like waves,” Li Du, an MIT physicist and lead author of the study, told Live Science. “Basically they are quantum mechanical objects.”
The interactions between these isolated systems are particularly important for understanding quantum phenomena such as superconductivity and overexposure. But the strength of these interactions usually depends on the separation distance, which can pose practical problems for researchers studying these effects; their experiments are limited by how close they can bring the atoms.
“Most atoms used in cold experiments, such as the alkali metals, need to be in contact to interact,” Du said. “We are interested in dysprosium atoms, which are special [in that they] can interact with each other over long distances through dipole-dipole interactions [weak attractive forces between partial charges on adjacent atoms]. But even though there is this long-range interaction, there are still some kinds of quantum phenomena that cannot be realized because this dipole interaction is so weak.”
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Bringing in cold atoms in close proximity while maintaining control over their quantum states is a significant challenge, and until now experimental limitations have prevented researchers from fully testing theoretical predictions about the effects of these quantum interactions.
“In ordinary experiments, we trap atoms with light, and this is limited by the diffraction limit — on the order of 500 nanometers,” Du said. (For comparison, a human hair is between 80,000 – 100,000 nanometers wide, according to National Nanotechnology Initiative.)
Using a laser beam focused through a lens, researchers can create a “Gaussian focal point,” which is like an energy well in the laser beam that traps certain atoms in position. This is known as optical tweezers, but the size of the tweezers (the width of the energy well) is limited by the wavelength of the laser light. This minimum width is called the diffraction limit.
Du’s team devised a clever trick to overcome this diffraction limit by exploiting another quantum property of dysprosium atoms: their spin. Atomic spin can point up or down – but most importantly, they have slightly different energies. This means the team can use two different laser beams with slightly different frequencies and polarization angles to separately capture the up and down spin of the dysprosium atoms.
“If atom A doesn’t see light B and atom B doesn’t see light A, they basically have independent control,” he explained. “Because the atoms are always right in the center of the Gaussian beam, you can move [the two different trapped particles] arbitrarily close.” By carefully controlling the two optical tweezers, Du’s team brought the up-spinning and down-spinning dysprosium atoms to within 50 nanometers of each other, increasing the interaction strength by 1,000 times from 500-nanometer levels.
With this double layer established, the team began a series of experiments to study quantum interactions at close range. They heat one of the dysprosium layers completely separated from the other by a vacuum gap. Incredibly, they observed heat transfer to the second layer through the void space.
“Usually you need contact or radiation to transfer heat, which we don’t have here,” Du said. “But we still see heat transfer, and it must be due to long-range dipole-dipole interactions.”
A seemingly impossible heat transfer was just one of the strange effects the team investigated. They are now eager to further explore the potential of quantum interactions on this scale. The group is now beginning to study how these bilayers interact with light. But Du is particularly interested in another quantum effect called Bardeen-Cooper-Schrieffer (BCS) pairing—a quantum bound state experienced by certain subatomic particles called fermions at low temperatures.
“The BCS pairing between the layers is very important for superconductivity,” he said. “A few years ago, a theoretical paper predicted that if we had this kind of bilayer system connected by long-range dipole-dipole interactions, you could form a BCS pair. We had not been able to see this experimentally before, but now it may be possible with our system.”